A marine muffler adapted with an internally tunable sound suppression structure is adaptable to different configurations thereby allowing for structural adjustments to optimize sound suppression and minimize backpressure for particular engine and exhaust conditions. A muffler housing defines an interior volume and includes an exhaust inlet and outlet. The outlet has a reduced diameter as compared with the inlet such that the exhaust gas accelerates through the muffler. An inclined baffle partitions the interior volume into a lower chamber in communication with the exhaust inlet and an upper chamber in communication with the exhaust outlet. Exhaust ducts attached to each side wall penetrate the baffle thereby placing the upper and lower chambers in fluid communication. A corrugated partition divides each exhaust duct into a plurality of non-circular conduits. The muffler may be tuned by selectively capping one or more conduit outlets.